1. Field of the Invention
This invention relates to a method for performing a blood count by means of a blood smear.
2. Discussion of the Art
Automated counting of blood cells typically involves counting blood cells after a sample of whole blood having a known volume is obtained and subsequently diluted in an appropriate diluent. Knowledge of the initial volume of the sample and the degree of subsequent dilution allows a quantitative determination of the numbers of different types of cells in the given volume of the original sample of whole blood. For example, if a microliter of whole blood is diluted so as to yield a volume of 1000 microliters, the dilution ratio is said to be 1:1000, and the dilution factor is said to be 1000. If a blood count for this diluted sample of blood indicates that there are 5000 red blood cells per microliter, the red blood cell count in the original undiluted blood sample is equal to the product of 1000 and 5000, i.e., 5,000,000. Thus, the actual blood count of the undiluted sample is 5,000,000 red blood cells per microliter.
Several physical methods for detecting and enumerating blood cells have been employed, such as, for example, analysis of the impedance characteristics of the blood cells by means of either direct current or radio frequency signals, the use of optical flow cytometry, wherein cells, which are either stained or in their near native state, are examined by means of light scatter characteristics, absorbance characteristics, fluorescence characteristics, or any combination of the foregoing. It has also been suggested that blood cells can be quantified by means of direct imaging of the blood cells in combination with analysis of microscopic images of the blood cells via flow cytometry or while the blood cells are suspended in a chamber having specified dimensions. Instruments have been developed in which either diluted or undiluted samples of blood can be introduced into a counting chamber, the dimensions of which are known, and a blood count can be generated by analysis of digital images. All of these approaches can be used to generate the parameters of a blood count.
After a blood count has been completed by one of the aforementioned methods, a number of the blood samples typically require additional analysis by means of a process that involves preparation, staining, and examination of a blood smear. The process of analyzing a blood smear can employ a variety of techniques, including manual, automated, or semi-automated techniques. The analysis of a blood smear can be used to confirm the accuracy of a blood count, to detect potential interfering substances, and to detect some of the fine sub-cellular features of cells that cannot be detected or interpreted by conventional analyses of a blood count.
Blood cells are not homogeneous. Blood cells contain sub-cellular features that are smaller than the cells themselves. Such sub-cellular features include nuclei, nucleoli, granules, and cell membranes. Particular examples of analyses of sub-cellular features include examination of the shapes of the red blood cells and variations in the shapes of the red blood cells. For example, it is possible to determine the ratio of the size of the nucleus of the cell to the size of the cell itself by measuring the cross sectional area of each (i.e., the nucleus of the cell and the cell itself) and dividing the measured values. This ratio, and various other parameters, can be used to determine the degree of normality of a blood cell.
Potential interfering substances include, but are not limited to, sickle cells, lyse-resistant red blood cells, cells that aggregate for various reasons, nucleated red blood cells, and unusually high lipid concentrations. Generally, these interfering substances are abnormalities in the structure(s) of blood constituent(s), which abnormalities alter the normal reflective and absorptive characteristics of blood constituents, which normal characteristics enable the measurements of blood parameters.
With respect to analysis of a blood smear, after a blood smear is prepared, the blood smear can be stained by means of at least one appropriate stain to identify the morphological characteristics of the blood cells and sub-cellular features of the blood cells. The process of identification can be manual or automated. Typically, a stained blood smear is examined by a human morphologist, who subjectively assesses the morphological appearances of the cells to provide either quantitative impressions of the proportions of different leukocytes or semi-quantitative impressions of the degree of morphological abnormality. Attempts have also been made to automate the process of analyzing a blood smear by means of automated microscopes and software to recognize patterns in digital images to not only classify leukocytes but to also provide an interpretation of the morphological changes.
Thus, the performance of a blood count and the subsequent morphological analysis of a blood sample require discrete steps that may involve processing the sample of blood through an automated blood counting device, forming and staining of a blood smear of the blood sample, either manually or by means of an automated device, followed by morphological review of the stained blood smear, either manually or by means of an automated device.
Although the practices previously described are in widespread use, and although the semi-quantitative assessment of cells is possible by a morphological review, performing a quantitative complete blood count on a blood smear has never been suggested. Such a process has two inherent limitations. When a sample of blood is spread to form a blood smear, the volume of blood used to form the blood smear cannot be sufficiently controlled to a point where an accurate estimate of the volume of blood can be made, with the result that the absolute number of cells present in the blood smear cannot be determined. Furthermore, although devices in which a monolayer of a blood sample can be deposited have been developed, these devices typically rely on centrifugation to distribute cells evenly across the surface of a rectangular-shaped microscope slide. In FIG. 1A, a microscope slide is designated by the reference numeral 10, and a drop of blood is designated by the reference numeral 12a. In FIG. 1B, the microscope slide is designated by the reference numeral 10, and the blood smear is designated by the reference numeral 12b. The arrow 14 represents the direction of rotation of the microscope slide 10 during the centrifugation process. Typically, some unknown volume of the blood sample is lost from the microscope slide during the centrifugation process. Because the quantity of cells lost is unknown and unpredictable, an accurate estimate of the volume of blood remaining on the microscope slide at the end of the analysis cannot be made. Therefore, only limited information can be derived with respect to the proportions of cells in the blood sample, and no information that requires knowledge of the total volume of the blood sample can be made. In effect, no measurements for determining the concentration of cells can be made.
There are two alternative approaches currently used for preparing blood smears. The first approach, which is not in widespread use, is the cover slip method. In this method, a drop of a blood sample is placed on a microscope slide. This drop is covered with a cover slip, and the blood smear is subsequently formed by moving the microscope slide and cover slip in opposite directions, thereby effectively smearing the sample. In FIG. 2A, a microscope slide is designated by the reference numeral 20, a drop of blood is designated by the reference numeral 22a, and a cover slip is designated by the reference numeral 24. In FIG. 2B, the microscope slide is designated by the reference numeral 20, the blood smear is designated by the reference numeral 22b, and the cover slip is designated by the reference numeral 24. The arrow 26 represents the direction of movement of the cover slip 24.
The second approach, which is much more widely used, is the wedge or push smear. In this method, a drop of a blood sample is placed on a first glass slide, typically a microscope slide. A second glass slide, which is termed a smearer or spreader, is first placed downstream of the drop of the blood sample and is then drawn back to the drop of the blood sample, whereby the drop of the blood sample is spread across the line of contact between the drop of the blood sample and the second glass slide. The second glass slide, i.e., the spreader, is then propelled forward, i.e., in the downstream direction, in a single rapid, but gentle, linear motion, whereby the drop of the blood sample is dragged behind the spreader, thereby forming a blood smear. See, for example, Automatic Working Area Classification in Peripheral Blood Smears Using Spatial Distribution features Across Scales, W. Xiong, et al.; LET'S OBSERVE THE BLOOD CELLS, D. Tagliasacchi, et al., April 1997, incorporated herein by reference. In FIG. 3A, a first glass slide is designated by the reference numeral 30, a drop of blood is designated by the reference numeral 32a, and the second glass slide, i.e., the spreader, is designated by the reference numeral 34. In FIG. 3B, the first glass slide is designated by the reference numeral 30, and the blood smear is designated by the reference numeral 32b. The arrow 36 represents the direction of movement of the second glass slide 34. In the resulting blood smear, the blood sample is deposited on the first glass slide in a wedge in which the thick end of the wedge is positioned at the point of initial contact of the drop of the blood sample on the first glass slide, and the thin end of the wedge, which is positioned downstream of the thick end of the wedge, contains a monolayer of cells. However, the wedge or push smear requires that the morphological analysis be confined to the area of the blood smear in which the cells are distributed very thinly in a true monolayer or in a near monolayer. In FIG. 4, a microscope slide is designated by the reference numeral 40. The thick portion of the blood smear is designated by the reference numeral 42, the thin portion of the blood smear is designated by the reference numeral 44, and the part of the blood smear suitable for counting cells, i.e., the true monolayer or near monolayer, is designated by the reference numeral 46. In the thick portion of the blood smear, the cells may overlay one another to such an extent that an automated instrument or a human morphologist is unable to reliably identify and record the morphology of the cells. Cells distributed in the upper layers tend to occlude the two-dimensional images of the cells in the lower layers, when the cells are viewed from above. To an observer, when the edges of cells overlap, the multiple layers of cells appear as a single large, irregularly shaped area. For example, two-dimensional imaging algorithms have difficulty in discerning the difference between two small overlapping cells and one larger cell having an irregular shape. This problem appears to negate the ability to perform a quantitative analysis of the numbers of leukocytes, erythrocytes, and platelets in a blood smear, because the area of a blood smear that is suitable for cell counting would vary unpredictably from blood sample to blood sample with respect to the thickness and length of the blood smear. Such variations are shown in FIGS. 5A, 5B, 5C, and 5D. In FIG. 5A, the slide is designated by the reference numeral 50a, and the blood smear is designated by the reference numeral 52a; in FIG. 5B, wherein the blood smear exhibits a difference in shape from the blood smear shown in FIG. 5A, the slide is designated by the reference numeral 50b, and the blood smear is designated by the reference numeral 52b; in FIG. 5C, wherein the blood smear exhibits a difference in length from the blood smear shown in FIG. 5A, the slide is designated by the reference numeral 50c, and the blood smear is designated by the reference numeral 52c; in FIG. 5D, wherein the blood smear exhibits a difference in breadth from the blood smear shown in FIG. 5A, the slide is designated by the reference numeral 50d, and the blood smear is designated by the reference numeral 52d. In summary, even if the same volumes of blood samples were used to form blood smears, the areas being evaluated for counting cells would differ from sample to sample. The principal factor for determining the thickness and length of a blood smear would likely be the overall viscosity of the sample, which, in turn, is likely to be determined primarily by the concentration of hemoglobin in the sample.
Additional information relating to methods for examining blood smears can be found at, for example, Peripheral Blood smear—Clinical Methods—NCBI Bookshelf, Clinical methods, The History, Physical, and Laboratory Examinations, Third edition, H. Kenneth Walker, W. Dallas hall, J. Willis Hurst, Butterworths, Peripheral Blood Smear, Edward C. Lynch, Hematology Laboratory: Proper Preparation of a Peripheral Blood Smear, Slide Staining with Wright's Stain; Now peripheral blood smears preparation doesn't depend on laboratory technician's mastery, Scientific and practical magazine >No. 6, February, 2005: yahoo answers, and Evaluation of the Blood Smear, M. Christopher, University of California Davis, Department of Pathology, Microbiology and Immunology School of Veterinary Medicine, Davis Calif., USA, all of which are incorporated herein by reference.